A typical paper-making machine for coated papers comprises in sequence: a Foudrinier wire; a press section; a main dryer section; a size press or on-machine coater; a further dryer section; and a reel-up section. The feedstock is applied to the wire upon which the web is formed and passes into the press section in which a substantial quantity of water is removed from the web. From the press section the still very wet web passes through the rolls of the main dryer section from which it emerges with a relatively low moisture content. The paper then passes into a size press or coater to receive its surface coating, and then passes through a further dryer to the reel-up roll.
If a non-coated paper is being manufactured then the size press or on-machine coater and the further drying section are omitted. In some cases the non-coated paper produced in this way is coated on a separate machine having one or more coating stations and dryer sections.
In some paper manufacture the main dryer section may be divided into two parts with an intermediate pair of smoothing rolls between the two parts.
The paper emerging from the main dryer section has a moisture content (water percentage by weight) of typically 5-10% dependent on the product and the moisture content of the paper as finally reeled-up is also typically in this range. A heavy paper has a grammage (weight in grams per square meter) of say 100 so that the grammage of water is up to 10 for the quoted moisture content. Clearly a much heavier product such as board would have a correspondingly higher grammage of water at the same moisture content.
It is conventional to measure moisture content on leaving the main dryer or at reel-up and such measurement may be used to adjust the machine operation toward achieving desired parameters. One technique for measuring moisture content is to utilize the absorption spectrum of water in the infra-red which exhibits a relatively sharp absorption waveband at a wavelength of 1.94 microns (.mu.m). Monitoring or gauge apparatus for this purpose is commonly in use. Such apparatus conventionally uses either a fixed gauge or a gauge mounted on a scanning head which is repetitively scanned transversely across the web at the exit from the dryer section and/or upon entry to reel-up, as required by the individual machines. The gauges use a broad-band infra-red source and one or more detectors (about which more is said below) with the wavelength of interest being selected by a narrow-band filter, for example an interference type filter. The gauges used fall into two main types: the transmissive type in which the source and detector are on opposite sides of the web and, in a scanning gauge, are scanned in synchronism across it, and the scatter type (sometimes called "reflective" type) in which the source and detector are in a single head on one side of the web, the detector responding to the amount of source radiation scattered from the web. It is important to note that the signal sensed by the detector is essentially a measure of water content, i.e. grammage of water. It does not give moisture content directly. The measurement at 1.94 .mu.m is essentially due to water alone; it is not affected by the paper itself which is mainly cellulose.
To improve the operation of both types of gauge it is also the practice to ratio (or perform other compensating calculation of) the detector signal at 1.94 .mu.m against a reference signal generated by detection at a wavelength at which water has little or no absorption of infra-red. As mentioned the absorption band at 1.94 microns is sharp and conveniently the reference wavelength is chosen at a shorter wavelength of about 1.83 .mu.m (sometimes simply referred to as the 1.8 .mu.m reference). 1.7 .mu.m is used for the reference in some gauges. The reason for using the shorter rather than the longer wavelength side of the absorption band will become clear later. Measurements are commonly expressed in a converse form, namely in terms of the % transmission (transmissivity) at the selected wavelength which is inversely related to the absorption.
Without going into detail of the factors that affect the absolute value of the measurement at the selected absorption band (1.94 .mu.m), it is sufficient to say that the ratio technique provides compensation for many factors that would otherwise produce errors in a measurement at a single wavelength.
The translation of the measurement of water content to one of moisture content requires a measure of the weight (grammage) of the web, either the paper or paper plus water, to which the water content can be related. Such a measurement can also be made with an infra-red gauge in that the infra-red absorption spectrum of cellulose, which is the major constituent of paper, has a well-defined absorption band at a wavelength that is adjacent and longer than that of the water absorption band. This wavelength is about 2.1 .mu.m so that by use of an appropriate narrow-band filter at this wavelength a third detection signal can be obtained using the cellulose content as a measure of paper weight. As with the water measurement this cellulose absorption signal is preferably ratioed against a reference and in practice the 1.83 or 1.7 .mu.m reference can serve for both, since at both wavelengths there is little absorption by cellulose. It will be realised that the water content reference wavelength is chosen on the short wavelength side to avoid interference that would otherwise arise from the nearby cellulose absorption band if a reference measurement were made on the longer wavelength side. The selection of a reference wavelength for other materials would have to be made with regard to the absorption spectrum for that material including its moisture content.
Alternatively the grammage of the sheet may be measured by a separate gauge using some other technique, such as the absorption of .beta.-rays by the web. However measured, the grammage measurement is combined with the infra-red measurement of total water content by calculation to give the moisture content as a percentage, or as a ratio if preferred.
The statement just made that the response at 1.83 .mu.m is little or not affected by water content is subject to an important qualification that will now be explained.
Scanning infra-red gauges of both the transmissive and scatter type are well established and have various refinements beyond the basic functions discussed above. Examples of numerous such gauges are the scatter type gauge available from Measurex Corporation in the United States under the model No. 2238, and the transmissive type gauge available with the 8012 control system from Paul Lippke GmbH. & Co. KG. in West Germany.
Further discussion of various factors influencing the measurement of water content by infra-red gauges can be found in U.S. Pat. No. 4,006,358 (Howarth) which discloses measurement at 1.94 .mu.m against a 1.7 .mu.m reference; and in British patent specification 2,044,443 which discloses measurement at 1.94 .mu.m against a 1.8 .mu.m reference, together with a cellulose measurement at 2.1 .mu.m providing a correction value used in the calculation of moisture content. This specification is based on a proposition that the water content (weight per unit area) of a "film" is dependent on the absorption properties of other material in the film. Reference may also be had to a paper entitled "Improved Infrared Moisture Measuring Techniques" given by Peter G. Mercer at a symposium held at Maidstone, England on 11-12th Oct., 1978 by the Institute of Measurement and Control, and subsequently published by that Institute.
In making measurements at the various wavelengths mentioned it is the practice to use a narrow-band filter to select only the wavelength of interest. The bandwidth at 50% of peak transmission is typically 0.03 .mu.m. Thus as will be better appreciated from the subsequent description, the spectrum region between 1.83 and 1.94 .mu.m is relatively wide in terms of the filter bandwidths normally used and the wavebands are well separated.
British Patent Specification 1,013,171 includes a discussion on the tolerances acceptable in filters nominally operating at 1.94 .mu.m. The specification discusses how far from the nominal centre-wavelength the filters may depart but is speaking of wider filters of 0.08 .mu.m bandwidth where greater derivation from the nominal centre could be tolerated but is not preferred because of the loss of sensitivity to the wanted 1.94 .mu.m wavelength.
It is noted that this specification is referring to filters available some twenty years or more ago. With present technology there is no difficulty in obtaining filters accurate in both bandwidths and nominal centre-wavelength so that a nominal 1.94 .mu.m filter can be expected to be at that wavelength with a high degree of assurance.
Infra-red moisture gauges, of whatever type, have been customarily used at the dry end of paper machines, that is after the main drying section, where moisture content is relatively low, typically 5 to 10% for most grades of paper.
Recently there has been an increasing interest in measurement at the wet end of the machine. Typically, this involves a water to dry weight ratio (or "moisture ratio") of 4-6 (corresponding to a "moisture content" of 80-85.7%) in the web coming off the wire, and a moisture ratio of 1-3 following the press section. For a sheet with a dry weight of 100 grams/m.sup.2, the grammage of water in the first instance would be 400-600 and in the second instance 100-300. Thus a completely different order of magnitude of measurement is required.
It is considered that in future more attention will have to be given to water content at the wet-end in establishing better control of the paper-making process and, for example, in avoiding the waste of energy in drying off water in excess of that required for proper production. The control at the wet end requires an on-line measurement of moisture content (the web here being generally predominantly water by weight) and a means for making such measurements reliably at the high water grammage abovementioned.
It is generally considered by suppliers of infra-red moisture gauges that they cannot be reliably used for water contents actually in paper in excess of a grammage of 25. This view is supported in the Mercer paper mentioned above. As reported in that paper, work has been done on water film alone to show that current gauges can measure up to 10 times that grammage of water on its own, i.e. measurements on a film of water. However, this is not a practical measurement and the large discrepancy with the figure quoted for water found as moisture in paper is due to the fact that in paper the infra-red radiation does not follow a straight line path but is multiply scattered by the fibres in the paper. The total absorption is proportional to total path length which is greatly increased by the scattering within the web. The detector signal thus represents the absorption over mean path length.
The difficulty is that as the absorption increases the signal to the detector decreases so that the detection limit is set by the noise and interference associated with the detector operation in the inherently noisy environment of a paper-making machine.
Consequently there would be a substantial advance if an infra-red spectroscopic technique were available to raise the practical limit for the measurement of water content in paper toward the 250 g/m.sup.2 that has been quoted for pure water, let alone to the typical figure of 400-600 g/m.sup.2 found in a web leaving the wire.
The high absorption, and low transmissivity, consequent upon an attempt to measure high water contents at 1.94 .mu.m effectively means that the weak signal is lost in noise and interference which may be regarded as a black level saturation where the black level itself is highly dependent on the local environment. This is true even where, as is conventional, the infra-red signal is pulsed or chopped to enhance selectivity in the detection process.